Estimates of atmospheric composition resulting from the SRES emissions scenarios
are presented in TAR WGIChapters 3-5.
Information on CO2 and ground-level O3 concentrations
is given in Tables 3-2 and 3-9.
More detailed regional estimates of pollutant concentrations and deposition
of acidifying compounds based on these scenarios also are beginning to emerge
(e.g., Mayerhofer et al., 2000; see Section 3.4).

To interpret the possible range of global temperature and sea-level response
to the SRES scenarios, estimates have been made with simple models for all 35
of the quantified SRES scenarios (Table 3-9; see
also TAR WGIChapters 9 and 11).
Estimates of global warming from 1990 to 2100 give a range of 1.4-5.8°Csomewhat
higher than the 0.7-3.5°C of the SAR. The main reason for this increase
is that the levels of radiative forcing in the SRES scenarios are higher than
in the IS92a-f scenarios, primarily because of lower sulfate aerosol emissions,
especially after 2050. The temperature response also is calculated differently;
rather than using the conventional idealized, equilibrium climate sensitivity
range of 1.5-4.5°C (IPCC, 1996a), the simple model is tuned to the
effective climate sensitivities of a sample of individual AOGCMs (see TAR
WGIChapter 9 for details). Sea-level rise between
1990 and 2100 is estimated to be 9-88 cm, which also accounts for uncertainties
in ice-melt parameters (see TAR WGIChapter
11).

3.8.3. Implications of SRES Scenarios for Regional Mean
Climate

3.8.3.1. Regional Information from AOGCMs

Estimates of regional climate change to 2100 based on AOGCM experiments are
described in TAR WGIChapters 9
and 10. The results of nine AOGCMs run with the A2 and
B2 SRES scenarios5 display many similarities with previous runs that assume
IS92a-type emissions, although there also are some regional differences (see
below). Overall, rates of warming are expected to be greater than the global
average over most land areas and most pronounced at high latitudes in winter.
As warming proceeds, northern hemisphere snow cover and sea-ice extent will
be reduced. Models indicate warming below the global average in the North Atlantic
and circumpolar southern ocean regions, as well as in southern and southeast
Asia and southern South America in June-August. Globally there will be
increases in average water vapor and precipitation. Regionally, December-February
precipitation is expected to increase over the northern extratropics and Antarctica
and over tropical Africa. Models also agree on a decrease in precipitation over
Central America and little change in southeast Asia. Precipitation in June-August
is projected to increase in high northern latitudes, Antarctica, and south Asia;
change little in southeast Asia; and decrease in Central America, Australia,
southern Africa, and the Mediterranean region.

The main differences between the SRES-based and IS92-based runs concern greater
disagreement in the SRES runs on the magnitude of warming in some tropical and
southern hemisphere regions and differing intermodel agreement on the magnitude
of precipitation change in a few regions, possibly as a result of aerosol effects.
However, there are no cases in which the SRES and IS92a results indicate precipitation
changes of opposite direction (see TAR WGIChapter
10).

3.8.3.2. Regional Climate Characterizations

Only a limited number of AOGCM results based on the SRES emissions scenarios
have been released and analyzed to date (i.e., results for the A2 and B2 scenarios),
and none were available for the impact studies assessed in this report. In the
meantime, alternative approaches have been used to gain an impression of possible
regional changes in climate across a wider range of emissions scenarios. One
method uses results from existing AOGCM simulations and scales the pattern of
modeled regional climate change up or down according to the range of global
temperature changes estimated by simple climate models for different emissions
scenarios or assumptions about climate sensitivity (Santer et al., 1990;
Mitchell et al., 1999; see also detailed discussion in TAR
WGIChapter 13). This "pattern-scaling"
method has been employed by Carter et al. (2000), using results from
simulations with seven AOGCMs, all assuming a radiative forcing approximating
the IS92a emissions scenario (for GHGs but excluding aerosols) scaled across
a range of global temperature changes estimated by using a simple climate model
for the four preliminary marker SRES emissions scenarios.

Regional-scale summary graphs of scaled temperature and precipitation changes
were constructed for 32 world regions, at subcontinental scale, chosen to represent
the regions being assessed by Working Group II (Carter et al., 2000).
Examples of individual plots are shown in Figure 3-2.
Changes are plotted alongside estimates of "natural" multi-decadal
variability of temperature and precipitation, extracted from two multi-century
unforced AOGCM simulations. The graphs thus provide a quick assessment of the
likely uncertainty range and significance of each AOGCM projection; they also
show the extent to which different AOGCMs agree or disagree with regard to regional
response to a given magnitude of global warming. Although a preliminary comparison
of these results with SRES AOGCM runs (which also include aerosol forcing) suggests
broad agreement on regional temperature and precipitation changes, more rigorous
comparison remains to be carried out, offering a useful test of the pattern-scaling
method.